A conventional synchronous torque coupler transmits rotational torque from one shaft to another. Such a coupler includes two rotors which are coupled to respective shafts. Permanent magnets are attached to each of the rotors. The torque coupler relies upon the tendency of opposite poles of permanent magnets on the two rotors to align to a position of minimum reluctance. For examples of conventional torque couplers, see U.S. Pat. Nos. 4,013,384 and 4,115,040. See also U.S. Pat. No. 3,890,515 (an excitation winding magnetizes salient ferromagnetic polar projections on the two rotors, which causes polar projections to align in a minimum reluctance magnetic circuit). For conventional synchronous torque couplers, the maximum change in magnetic circuit reluctance that can be realized is proportional to the height of the poles as compared to the non-pole space between the poles. The minimum obtainable reluctance is limited by the unity permeability of the space occupied by the non-poles.
When one rotor is rotated (the "driving" rotor) by external means such as a motor, the other rotor (the "driven" rotor) follows. In a synchronous reluctance coupling, a precise angular displacement is maintained between the two magnetically coupled rotors, with the driving rotor leading the driven rotor in phase by a few degrees. This angular displacement, or phase angle, between the two rotors is dependant upon the torque and the field, but it is independent of the speed of the rotors. Synchronism between the rotors is maintained up to a precisely discernible phase angle at which a maximum or pullout torque occurs. Above this angle, synchronism is lost. Between zero and the pullout angle, the phase angle increases with the applied torque, but is inversely proportional to the magnetic field. For any given steady-state torque, not exceeding the pullout angle, there is no energy expenditure in the rotors, mechanical losses due to windage and bearing friction excepted. Once synchronism is lost, a pulsating torque with an average value of zero is developed.
The loss of synchronism within a torque coupling device having permanent magnets in both the outer and inner rotors causes demagnetization of the permanent magnets, resulting in failure of the device. Similarly, a torque coupling device utilizing magnetic fields generated by electrical windings will be subject to excessive back EMF within the windings, leading to winding burnout and failure.
A distinction should be noted between synchronous reluctance coupling devices and coupling devices based upon either the hysteresis principle or the induction principle. These other coupling devices develop torque as a consequence of energy losses, e.g., heating, in one of the rotors. The torque is proportional to the quotient of the rotor loss divided by the slip speed. The induction, or eddy current coupling device cannot develop torque when the rotors are running synchronously. The hysteresis coupling device can develop a reduced torque at synchronism; however, the precise phase relationship between the rotors is indeterminate, depending on the previous torque history of the device.